Three main motor types dominate today's motor market: 1) AC Induction motor; 2) Brush-type DC motor; 3) Brushless DC motor. Each of these will be discussed in some detail to highlight distinctive characteristics of the DC motor of the present invention.
AC Induction Motor:
The AC induction motor may be modeled essentially as a rotating transformer where the primary coil represents the stator and the rotor serves as the secondary coil comprising one turn. Acting as a transformer, rotor (secondary) current is the result of voltage induced by a changing magnetic field created by the stator (primary). Stator and rotor are linked magnetically by an iron core (magnetic circuit) that conducts magnetic flux through both the rotor and stator windings. Ideally this magnetic circuit should offer little impedance to magnetic flux, in other words, be of high permeability and low reluctance. Actual reluctance in a real machine requires magneto-motive force (MMF) to drive flux through the magnetic circuit in much the same way that electromotive force (EMF or voltage) is required to drive electricity through an electric circuit. Electrical energy for magnetizing the core is added and removed to the magnetic circuit repetitively with each electrical half-cycle. Because it produces no work, either as heat or mechanical shaft power, this circulating energy is referred to as reactive power and the associated current as reactive current or magnetizing current.
Purely reactive current produces no real work (heat or mechanical) because it is 90° out of phase with the voltage. However, in a real coil with electrical resistance, the mere passage of reactive current through copper coils of finite resistance does generate heat as an unavoidable consequence of magnetizing the core. Magnetizing current is therefore not only reactive, but in a resistive circuit has a real component exactly in phase with voltage indicating actual power dissipation in the form of heat in the windings.
Obviously it is advantageous to reduce core reluctance in an AC induction motor in order to minimize reactive current and its associated heat generation. An appreciable source of reluctance in an AC induction motor is the rotor-stator gap. Maintaining a minimal gap limits the reactive current for cooler operation and acceptable motor efficiency.
Common to all transformers, including the AC induction motor, is the principle of flux cancellation. The magnetizing (reactive) primary current alone represents a small fraction of the maximum current capacity of a transformer/motor. Yet this relatively low primary current is all that is required to bring the core to saturation. High current flows through a transformer only when the secondary is also conducting. Primary current then is equal to both secondary and magnetizing currents. Except for the magnetizing current, which is constant for a given flux density and primary applied voltage, the additional magnetic fields produced by both primary and secondary currents completely cancel one another leaving only the constant core field produced by the magnetizing current. Flux cancellation permits a transformer or an AC induction motor to accommodate current levels tens of times higher than the magnetizing current alone yet without saturating the core.
The typical 3-phase stator of an AC induction motor creates a rotating magnetic field diametrically traversing the rotor. The rotational rate (synchronous frequency) of this field is determined by the frequency of the 3-phase electric power supply. Rotor rotation at synchronous frequency provides no relative motion between the rotating stator field and the rotor conductors. As rotor speed drops below synchronous frequency, i.e., begins to “slip”, the stator field then begins to move relative to the rotor conductors and thereby induces a voltage in the rotor circuit. The resulting rotor current produces a rotor field rotating relative to the rotor at slip frequency WSLIP, which is the difference between synchronous and rotor frequencies. This slip frequency adds to the shaft frequency to give a total rotor frequency, as seen from a stationary frame of reference, equal to the synchronous frequency of the stator. Thus both rotor and stator magnetic fields “lock in” at the same frequency as seen from a stationary reference point.
Rotor and stator fields are angularly displaced from one another by the so-called torque angle. Ideally the torque angle should be 90° to produce the highest torque possible per unit amp-turns or per unit heat generation. In practice the torque angle in an AC induction motor is significantly greater than the optimum 90° which leads to far less torque generation than would otherwise be possible. Because there is no physical connection of the rotor electrical circuit to the outside world there is no way to influence when current flows in the rotor with respect to stator current. Coordinating both rotor and stator fields is beyond direct control inasmuch as it is governed only by the inherent electromagnetic properties (reactance and resistance) of the rotor core/conductor circuit.
Two electromagnetic properties of the rotor determine when rotor current and its resulting magnetic field occur relative to the stator field: rotor resistance and rotor reactance. Resistance is an electrical property of the rotor conductors; reactance is a function of frequency and magnetic core properties. Without reactance, rotor current could flow with correct synchronization to the stator field to produce a 90° torque angle independent of slip. The unavoidable presence of rotor reactance, however, causes rotor current to lag behind the ideal timing to produce a torque angle greater than 90°, about 160–170°, approaching 180° where torque drops to zero. Because reactance is the product of rotor inductance and slip frequency, the effects of reactance increase with slip frequency. Yet slip is essential for inducing rotor current that produces torque. But slip also causes the rotor field to lag behind the stator field which reduces torque.
Thus two conflicting phenomenon are operative simultaneously. Initially, at low slip and low reactance, torque production approaches the ideal 90° torque angle. As slip frequency increases, a point is reached where the counter-productive effects of reactance begin to dominate and torque drops off regardless of increasing slip, a situation known as pull-out, the peak torque available.
Thus the AC induction motor has a definite peak in torque production that is impossible to surmount under any condition. Peak torque of the motor is considerable, however, as much as four times the rated continuous torque. But this high torque level is sustainable only for brief periods because of low efficiency and high heat generation due to the very poor torque angle.
In order to adjust the balance of opposing effects in favor of an improved torque angle at high slip, the rotor resistance may be raised without affecting its reactance Thus higher slip may be realized before reaching peak torque. This technique of varying the rotor resistance was used in early variable speed strategies before the advent of power electronics. However, raising rotor resistance also raises rotor losses making this method of speed control very inefficient, the efficiency being approximately:Efficiency=(1−S), where S=Wslip/Wsyn, where Wslip=slip frequency; and Wsyn=synchronous frequency
FIG. 1 is a phasor diagram for a conventional AC induction motor showing the rotor and stator currents IROT and ISTAT respectively. Their large magnetic fields (not shown) cancel one another leaving only the net magnetizing current and its net field BNET near core saturation BSAT which vector is always perpendicular to VSTAT and VROT as shown in the phasor diagram. Rotor current interacts with the net field to produce rotor shaft torque and stator reaction torque. Induced rotor voltage VROT is produced by stator field BSTAT slipping around the rotor at slip frequency WS. VROT in turn drives rotor current IROT to produce the rotor magnetic field (not shown). Both stator voltage VSTAT and rotor voltage VROT are supported by the same net magnetizing field BNET, the former at line frequency and the latter at slip frequency. Notice that the rotor voltage VROT appears as two normal components, one across the rotor winding resistance as VW and the other across the rotor reactance as VL, where VL is proportional to slip frequency WS and is expressed as VL=IROT×ROT=IROTWSLROT where LROT is the intrinsic rotor inductance. At low slip frequency, VL is very small compared to VW and torque angle θ approaches the ideal 90°. As slip frequency increases, VL becomes large with respect to VW and torque angle θ increases to values tending to reduce torque despite rising rotor current due to higher VROT at increased slip. Thus a point is reached where the poor torque angle overwhelms any increase in rotor current as slip increases further, after which torque begins to decline no matter how great the slip. The frequency where torque falls off with increasing slip is known as the pull out speed and the associated peak torque is referred to as pull-out torque.
In conclusion, the AC induction motor has innate uncontrollable characteristics which ultimately prevent actualizing its full torque potential at high efficiency.
Brush-Type DC Motor:
The brush-type DC motor relies on an operating principle that does not invoke transformer action for flux cancellation. Rather, it incorporates salient (protruding) stator poles which effectively complete the stator magnetic circuit at low reluctance through the rotor while opening the rotor magnetic circuit to high reluctance through the stator. Thus with relatively little consumption of power (about 5% of total motor power), a small stator current (amp-turns) can provide a large magnetic field through which rotor conductors traverse to generate torque and rotor back EMF. On the other hand, large rotor currents produce a negligible stator field due to the high reluctance circuit offered to the rotor MMF by the salient poles. Consequently, there is negligible back EMF generated in the stator. This asymmetrical design prevents core saturation at high rotor current while permitting full flux density from low stator amp-turns.
Only a fraction of rotor copper is involved in conducting current at any particular moment, a natural consequence of attempting to reduce harmonic currents by creating a nearly ripple-free back rotor EMF that matches the DC power supply. But the penalty incurred is excessive rotor heating which limits peak torque production on a sustained basis at reasonable efficiency.
Aside from low running speed and poor utilization of rotor copper, the brush-type DC motor does indeed operate at an optimum torque angle and near the flux saturation limit. Together these two traits have imparted a reputation of extraordinarily high torque for traction applications such as in Diesel electric locomotives for example.
While excelling as a torque machine, the brush-type DC motor suffers from the electro-mechanical restrictions of mechanical brush commutation. While torque output is high, actual power and efficiency are not especially impressive due to low speed and ineffective copper utilization. Unlike the AC induction motor, brief spurts of enormous torque are available from a brush-type DC motor. There is no pull-out, no theoretical limit to the magnitude of momentary torque impulses.
Brushless DC Motor:
The brushless DC motor essentially turns the brush-type DC motor inside out by putting the “armature” on the outside (stator) while the permanent magnet (PM) rotor provides a constant DC magnetic field. Similar to the brush-type DC motor, it also operates at an optimal 90° torque angle.
Permanent magnet material has very low permeability which offers high reluctance to the stator field enabling high stator current without saturation. However, PM material has the ability to overcome its own internal reluctance to create a substantial field throughout the surrounding magnetic core. Consequently the same asymmetric magnetic conditions exist in the brushless DC motor as in a brush-type design except that no external electrical supply is necessary to produce the DC rotor field.
Permanent magnet materials are not presently available that approach the flux density of electromagnets. In fact, a flux density less than half of saturation is obtained from the best PM materials known. In addition, their temperature sensitivity renders motor applications problematic at sustained high power levels. Inasmuch as torque is the product of amp-turns and flux density, reducing flux density to half the saturation level by using PM materials requires twice the amp-turns to compensate and achieve the same torque. But twice the amp-turns results in quadrupling of heat generation per unit torque as compared to the present invention or brush-type DC motor.
Off-setting this torque deficit, as compared to the brush-type machine, the brushless DC motor is capable of very high rotational speeds so that its actual power may exceed that of a brush-type motor. Furthermore, the brushless motor is similar to brush-types inasmuch as very high momentary torque impulses are possible without pull-out.